It is important for an optical amplifier used in a wavelength-multiplexed communication system to have a uniform or flat gain spectrum. An EDFA can produce gain over a spectral width of more than 30 nm, but even under optimum pumping conditions the gain spectrum may not be uniform. A gain flattening filter (GFF) is known to be useful in optical amplifiers to reduce the variation in the gain over some band of wavelengths (see e.g., M. Tachibana, R. I. Laming, P. R. Morkel, and D. N. Payne, "Erbium-doped fiber amplification with flattened gain spectrum," IEEE Photonics Technology Letters, vol. 3, pp. 118-120, 1991). Static GFFs, however, can only provide optimum gain-flatness at a single gain value (i.e., gain at any particular wavelength). If the gain of an EDFA is changed by changing the inversion (e.g., by changing the pumping power or signal power), the gain changes in a spectrally dependent manner pumping power or signal power (see, e.g., C. R. Giles and D. J. D. Giovanni, "Spectral dependence of gain and noise in Erbium-doped fiber amplifiers," IEEE Photonics Technology Letters, vol. 2, pp. 797-800, 1990 and J. Nilsson, Y. W. Lee, and W. H. Choe, "Erbium doped fiber amplifier with dynamic gain flatness for WDM," Electronic Letters vol. 31, pp. 1578-1579, 1995). As a result, if a conventional EDFA is used in an application where its gain needs to be different from the design gain of the amplifier, its gain spectrum will show excess normalized gain ripple ((maximum gain--minimum gain)/minimum gain) as calculated in the wavelength band of interest. An example of how this can be a problem is provided by an optically amplified fiber transmission system where one needs to support fiber spans shorter than those for which the amplifier is designed. It is typically impractical to have separate amplifiers custom designed for each fiber span. Therefore, one is either forced to have an amplifier with a distorted gain spectrum or to add enough loss to the system so that the design gain is actually needed from the amplifier. An optical attenuator intentionally added to a system for gain-flattening purposes will tend to increase the amount of noise that is added to the signal and require additional pump power relative to a similar cascade of amplifiers that have been redesigned to provide flat gain at the actual gain level needed. This performance loss can be reduced by placing the added optical attenuation between the gain stages of a multistage optical amplifier (see, e.g., Y. Sugaya, S. Kinoshita, and T. Chikama, "Novel configuration for low-noise and wide-dynamic-range Er-doped fiber amplifier for WDM systems," in Optical Amplifiers and their Applications, 1995 OSA Technical Digest Series, Vol. (Optical Society of America, Washington, DC) 158-161 and N. E. Jolley, F. Davis, and J. Mun, "Out-of-band electronic gain clamping for a variable gain and output power EDFA with low dynamic gain tilt," in Conference on Optical Fiber Communication, 1997 OSA Technical Digest series, Vol. 6, (Optical Society of America, Washington, D.C.) 134-135).
It is an object of the invention to utilize this approach of placing attenuation between gain stages in an amplifier that contains a GFF, while maintaining good optical performance. This is especially important for wideband EDFA's where the GFF needed may attenuate only parts of the gain spectrum. This approach is also applicable when a high attenuation GFF is used alone or with other attenuating optical elements.
It is well known that the impact that a GFF has on the performance of an optical amplifier can be reduced by properly inserting the GFF between two gain stages (see e.g., M. Tachibana, R. I. Laming, P. R. Morkel, and D. N. Payne, "Erbium-doped fiber amplification with flattened gain spectrum," IEEE Photonics Technology Letters, vol. 3, pp. 118-120, 1991). For a conventional line amplifier the negative impact of the GFF can be significantly reduced with proper filter placement. However, as amplifiers move toward wider bandwidths, the impact of the GFF becomes more significant for a number of reasons. Wider bandwidths tend to require GFFs with larger peak attenuations. As the peak attenuation of a filter increases, its negative impact on amplifier noise/output power performance will generally increase also. Wide bandwidth amplifiers also frequently make use of the short wavelength portion of the erbium gain spectrum (or "blueband") which roughly extends from 1525-1540 nm. It is typically harder to achieve optimum noise performance in this part of the spectrum since the intrinsic noise performance of the amplifying fiber is more sensitive to the local inversion. As illustrated below, these effects can compound each other in multistage amplifiers where the final power stages may have very low inversions.
More particularly, FIG. 1 schematically illustrates a three stage EDFA 10. An optical signal to be amplified enters the EDFA 10 at input port 11 and the amplified optical signal exits the EDFA at output port 12. The EDFA 10 includes three gain stages whose power gains are designated as G.sub.1, G.sub.2, G.sub.3. Each of the gain stages G.sub.1, G.sub.2, G.sub.3 comprises a pumped segment of erbium doped optical fiber. The erbium dopant provides optical gain for optical radiation propagating in the optical fiber segment. Alternatively, it may be possible for other elements besides erbium, such as the rare earth elements, to provide the appropriate gain.
In FIG. 1, T.sub.i is the net (linear) transmittance up to the ith gain stage. Therefore, T.sub.i is the product of the linear transmittance factors of lossy components and the gain factor for amplifying components. Thus, T.sub.i may be viewed as the power transmission coefficient (accounting for insertion loss) for all components with the indicated position relative to the gain stages, G.sub.i, i=1, 2, 3. The quantities T.sub.i and G.sub.i may be a function of wavelength. EQU F.sub.total =F.sub.1 /T.sub.in-1 /+F.sub.2 /T.sub.in-2 +F.sub.3 /T.sub.in-3 (1)
where F.sub.1 is the noise factor (linear units) of the first gain stage and T.sub.in-1 is the net linear power transmittance of all the optical components from the amplifier input to the beginning of the first gain stage. The symbols associated with the noise contributions of the other stages are analogously defined. For a high gain amplifier (G&gt;.about.20 dB) the minimum possible value of F.sub.1 is 2. One typically designs optical amplifiers such that the first term in Eqn. 1 dominates the total noise factor while operating with F.sub.1 as close to the quantum mechanical limit as possible (high population inversion). Because of the low inversions typically used in subsequent gain stages (high pump to signal power conversion efficiency is typically easiest for attaining a lower inversion), the only way to make the impact of these stages on the overall amplifier noise factor small is to make EQU T.sub.in-1 =T.sub.1 G.sub.1 +. . . +T.sub.i-1 G.sub.i-1 (2)
i.e., the net power transmission factor from the amplifier input to the beginning of the ith stage, as high as possible. This can typically be accomplished by using a high gain (.about.20 dB) in the first stage. Higher gains are difficult to attain with a single stage due to the buildup of amplified spontaneous emission. However, GFF peak attenuations for wide band amplifiers frequently approach or exceed 10 dB while a 10 dB gain dynamic range would require a (variable) optical attenuator with a peak attenuation &gt;10 dB (10 working range+minimum loss). Furthermore, the noise factor of the low inversion stages can be in a range from near 10 to larger than 10. If components with insertion losses such as these are immediately cascaded within the amplifier, their aggregate attenuation would result in low values for T.sub.in-i and the overall noise factor of the amplifier would be affected.
It is a further object of the invention to overcome this problem so as to improve the performance of a multistage EDFA.